Gate oxide film structure for a solid state image pick-up device

ABSTRACT

In a solid-state image pick-up device in which a photoelectric converting section formed on a semiconductor substrate and a gate oxide film of a transfer path of a charge coupled device (CCD) which is close to the photoelectric converting section are constituted by a laminated film comprising a silicon oxide film (SiO) and a silicon nitride film (SiN), the gas oxide film has a single layer structure in which at least an end on the photoelectric converting section side of the gate oxide film does not contain the silicon nitride film.

This application is a Divisional of application Ser. No. 10/438,865,filed on May 16, 2003, now U.S. Pat. No. 6,946,694, the entire contentsof which are hereby incorporated by reference and for which priority isclaimed under 35 U.S.C. § 120.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a solid-state image pick-up device anda method of manufacturing the solid-state image pick-up device, and moreparticularly to a solid-state image pick-up device having a structure inwhich a gate oxide film includes a silicon nitride film and a method ofmanufacturing the solid-state image pick-up device.

2. Description of the Related Art

In a solid-state image pick-up device in related arts, as shown in anexample of a sectional structure in FIG. 7, a photodiode section and acharge transfer section are formed in a p well 11 provided on thesurface of a semiconductor substrate 10, and electric charges generatedin the photodiode section are led to a transfer channel comprising ann-type impurity region 14 by applying a voltage to the charge transferelectrode of the charge transfer section, and are sequentially read. Inthe charge transfer section, the electric charges generated in thephotodiode section are led to the transfer channel comprising the n-typeimpurity region 14, and a gate electrode 18 to be a charge transferelectrode and reading electrode (hereinafter referred to as a readingelectrode) is formed through a gate oxide film having a three-layerstructure including a silicon oxide film 15, a silicon nitride film 16and a silicon oxide film 17 on the n-type impurity region 14.

Thus, the gate oxide film provided under the reading electrode of thesolid-state image pick-up device has a so-called ONO structure in whicha silicon nitride film to be a gate having a high breakdown voltage isinterposed between silicon oxide films. With this structure, a thin gateoxide film having a high breakdown voltage is essential in thesolid-state image pick-up device which is thin and is moremicrofabricated recently. The ONO structure is essential to a reductionin the thickness of a gate film.

In the solid-state image pick-up device having such a structure, when alight is incident on a pixel section, it is photoelectrically convertedin an n-type impurity region 12 so that a signal charge a is generatedand is moved to the transfer channel 14 when a reading pulse is appliedto the gate electrode 18 to be the charge transfer electrode and readingelectrode. On the other hand, a signal charge by generated in thevicinity of the surface of the substrate is accelerated by an electricfield through the reading pulse, and a part thereof is changed into ahot electron and is trapped into the silicon nitride film, therebycausing the aging of a reading gate voltage.

With the advance of the microfabrication of the device, the impurityconcentration of the n-type impurity region 12 tends to be increased dueto a reduction in a resistance so that the convergence of an electricfield is more increased on the end of the reading electrode. Moreover,since a gate length is reduced, the number of collisions of an electronis decreased so that the frequency of the generation of the hot electrontends to be increased. Consequently, the aging of a voltage to beapplied to the reading gate has become a serious problem.

In the such a solid-state image pick-up device, thus, there is a problemin that a gate structure which has a high breakdown voltage and can havea thickness reduced and a gate structure in which aging is not generatedby a hot electron have a trade-off relationship and both of them cannotbe satisfied at the same time.

SUMMARY OF THE INVENTION

In consideration of the actual circumstances, it is an object of theinvention to provide a thin solid-state image pick-up device having astability, a high reliability and a high breakdown voltage withoutgenerating aging by a hot electron.

Moreover, it is another object of the invention to provide a method ofmanufacturing a solid-state image pick-up device which can easily bemanufactured and has a high reliability.

[Means for Solving the Problems]

The invention provides a solid-state image pick-up device in which aphotoelectric converting section formed on a semiconductor substrate anda gate oxide film of a transfer path of a charge coupled device (CCD)which is close to the photoelectric converting section are constitutedby a laminated film comprising a silicon oxide film (SiO) and a siliconnitride film (SiN), wherein at least an end on the photoelectricconverting section side of the gate oxide film does not contain thesilicon nitride film.

According to such a structure, the gate oxide film of the transfer pathof the charge coupled device (CCD) which is close to the photoelectricconverting section does not contain the silicon nitride film on the endat the photoelectric converting section side. The silicon nitride filmis not present on the end of the electrode on which an electric fieldconverges most greatly in the application of a reading pulse. Even if asignal charge generated in the vicinity of the surface of the substrateis changed into a hot electron, consequently, it is possible to lessen atrap as compared with the structure in which the silicon nitride film isincluded. Accordingly, it is possible to maintain an excellent readingcharacteristic without causing the aging of a voltage to be applied to areading gate.

It is desirable that the laminated film should be a film having an ONOstructure in which a silicon nitride film (SiN) is provided in a siliconoxide film (SiO). By using the film having the ONO structure, it ispossible to constitute a gate oxide film which is thin and has a highbreakdown voltage, and the gate oxide film of the transfer path has asingle layer structure in which the silicon nitride film is notcontained on the end at the photoelectric converting section side.Therefore, the hot electron is less trapped into the silicon nitridefilm.

Moreover, it is desirable that the gate oxide film in a region whichdoes not contain the silicon nitride film should have a width of 0.2 μmor less. The silicon nitride film is not present in a region in whichthe hot electron is easily stored. Consequently, the hot electron can beprevented from being trapped. On the other hand, in some cases in whichthe width of 0.2 μm is exceeded, a breakdown voltage becomesinsufficient.

It is desirable that the photoelectric converting section should beconstituted by a photodiode, the transfer path should have a readinggate electrode which is close to the photodiode and a transfer electrodewhich is close to the reading gate and is independent, and an end of thesilicon nitride film provided under the reading gate electrode should bepositioned inward from an end of the reading gate electrode.

Also in the case in which the reading gate and the charge transferelectrode are provided independently, thus, the end of the siliconnitride film provided under the reading gate electrode is more retreatedthan the end of the reading gate electrode. Consequently, the hotelectron can be prevented from being trapped and the breakdown voltagecan also be maintained sufficiently.

Moreover, it is desirable that the gate oxide film provided under thereading gate electrode should have a single layer structure.Consequently, the silicon nitride film is completely removed under thereading gate electrode. Consequently, the hot electron is not trapped atall.

Furthermore, the invention provides a method of manufacturing asolid-state image pick-up device, comprising the steps of forming a gateoxide film having a lamination structure including at least a siliconnitride film and a silicon oxide film on an insulating film of a surfaceof a semiconductor substrate, forming a charge transfer electrode on thegate oxide film, etching the gate oxide film by using the chargetransfer electrode as a mask, and carrying out etching on an etchingcondition that an etching selective ratio of the silicon nitride film tothe silicon oxide film is high, thereby selectively removing an end ofthe silicon nitride film after the etching step.

According to such a structure, the charge transfer electrode issubjected to patterning and the gate oxide film is then subjected to thepatterning by using the charge transfer electrode as a mask.Consequently, a gate oxide film from which the silicon nitride film isremoved at the end can easily be formed without requiring an advancedlithographic technique. Accordingly, it is possible to provide asolid-state image pick-up device which has a high breakdown voltage anddoes not cause aging by a hot electron.

It is desirable that the selective removing step should be a chemicaldry etching (CDE) step. Consequently, it is possible to easily removethe silicon nitride film with a high controllability.

Moreover, it is desirable that the selective removing step should be anisotropic etching step using phosphoric acid. Consequently, it ispossible to easily remove the silicon nitride film without a highcontrollability.

Furthermore, it is desirable that the silicon nitride film should beremoved and the oxidizing step should be then carried out. Consequently,the region from which the silicon nitride is removed is covered with thesilicon oxide film. Thus, a solid-state image pick-up device having ahigh reliability can be formed very easily.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a solid-state image pick-up device according toa first embodiment of the invention,

FIGS. 2( a) to 2(e) show the views showing a process for manufacturingthe solid-state image pick-up device according to the first embodimentof the invention,

FIG. 3 is a view showing a solid-state image pick-up device according toa second embodiment of the invention,

FIG. 4 is a view showing a solid-state image pick-up device according toa third embodiment of the invention,

FIGS. 5( a) to 5(d) show the view showing a process for manufacturingthe solid-state image pick-up device according to the third embodimentof the invention,

FIG. 6 is a view showing a solid-state image pick-up device according toa fourth embodiment of the invention, and

FIG. 7 is a view showing a solid-state image pick-up device according toa related art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Mode for Carrying Out theInvention

Embodiments of the invention will be described below with reference tothe drawings.

First Embodiment

As shown in a schematic sectional view of FIG. 1, a solid-state imagepick-up device is characterized in that a gate electrode 18 to be acharge transfer electrode comprising a polycrystalline silicon film isformed through insulating films 15, 16 and 17 (hereinafter referred toas gate oxide films) having an ONO structure on the surface of a siliconsubstrate 10, and the gate oxide films provided under the gate electrode18 have a single layer structure in which the silicon nitride film 16 isnot present on an end at the photodiode section side.

Other portions are formed in the same manner as those in the solid-stateimage pick-up device according to the conventional example shown in FIG.7.

Next, a process for manufacturing the solid-state image pick-up devicewill be described with reference to FIGS. 2( a) to 2(e). In thisexample, ion implantation is carried out to form an n-type impurityregion 12 for forming a photodiode region, a p-type impurity diffusionregion 13 and an n-type impurity region 14 to be a transfer channel, anda gate oxide film and a gate electrode are then formed. In this case, onthe assumption that a diffusion length is increased by heating at asubsequent step, it is necessary to set a diffusion time. In thefollowing steps, a photodiode region and a transfer channel which are tobe formed in a semiconductor substrate will be omitted forsimplification.

As shown in FIG. 2( a), first of all, the silicon oxide film 15 having athickness of 15 nm, the silicon nitride film 16 having a thickness of 50nm and the silicon oxide film 17 having a thickness of 10 nm are formedin a p well 11 provided on the surface of the n-type silicon substrate10, and a gate oxide film having a three-layer structure is formed.

Subsequently, a high concentration doped polycrystalline silicon filmhaving a thickness of 0.4 μm for forming the gate electrode 18 isprovided on the gate oxide film.

As shown in FIG. 2( b), then, the polycrystalline silicon film issubjected to patterning through reactive ion etching by using, as amask, a resist pattern formed by photolithography, thereby forming thegate electrode 18 to be a reading gate. Furthermore, the gate oxidefilms are sequentially etched by using the gate electrode 18 as a mask.

As shown in FIG. 2( c), thereafter, thermal oxidation is carried out toform a thermal oxide film 19 on the surface of the reading gate. At thistime, the thermal oxide film 19 is rarely formed on the gate oxide film,that is, the silicon nitride film 16. Accordingly, removal can easily becarried out by an acidic processing.

As shown in FIG. 2( d), subsequently, the oxide film provided on thesilicon nitride film 16 is removed by the acidic processing, and thesilicon nitride film is selectively removed by wet etching using thermalphosphoric acid in a width of approximately 0.2 μm from a reading gateend.

Then, the thermal oxidation is carried out and a region from which thesilicon nitride film is removed is filled with a silicon oxide film asshown in FIG. 2( e).

The final step of forming the silicon oxide film is not restricted tothe thermal oxidation but a plasma CVD method and a low pressure CVDmethod may be used. Moreover, the processing may be exactly transferredto subsequent steps.

According to the solid-state image pick-up device thus formed, there isemployed a single layer structure in which the gate oxide film providedunder the reading gate which is close to the photodiode region does notcontain the silicon nitride film on the end, and the silicon nitridefilm is not present on the end of an electrode on which an electricfield converges most greatly in the application of a reading pulse. Evenif a signal charge generated in the vicinity of the surface of thesubstrate is changed into a hot electron, consequently, the hot electronis rarely trapped. Accordingly, it is possible to maintain an excellentreading characteristic without causing the aging of a reading gatevoltage.

Moreover, it is also possible to carry out formation without requiring anew photolithographic process by only carrying out isotropic etchingover an edge. Thus, a solid-state image pick-up device having anexcellent characteristic can be formed very easily with a highworkability.

Second Embodiment

Next, a second embodiment of the invention will be described withreference to FIG. 3. In the first embodiment, the silicon oxide film 15,the silicon nitride film 16 and the silicon oxide film 17 are formed onthe silicon substrate 10, and the polycrystalline silicon film to be thereading gate is formed thereon and is thus subjected to patterning, andthe silicon nitride is retreated by side etching using thepolycrystalline silicon film as a mask. In the embodiment, the siliconnitride is previously subjected to the patterning and a reading gate isprotruded therefrom to surround the edge of the silicon nitride film 16.

More specifically, the silicon oxide film 15, the silicon nitride film16 and the silicon oxide film 17 are formed and the silicon oxide film17 and the silicon nitride film 16 are subjected to the patterning, anda reading gate 18 is formed to be protruded from the edge of the siliconnitride film 16. Other portions are formed in the same manner as thoseof the first embodiment.

With such a structure, similarly, it is possible to prevent a readingvoltage from being raised by trapping a hot electron. There is a problemin that another photolithographic step is added in this case.

Third Embodiment

Next, a third embodiment of the invention will be described withreference to FIG. 4. In the second embodiment, the silicon oxide film15, the silicon nitride film 16 and the silicon oxide film 17 are formedon the silicon substrate 10, and the silicon oxide film 17 and thesilicon nitride film 16 are subjected to the patterning to form the gateelectrode 18 to be the reading gate so as to be protruded from the edgeof the silicon nitride film 16. In the embodiment, the gate oxide filmand the gate electrode are sequentially subjected to the patterning inthe same manner as in the first embodiment, and a sidewall 20 comprisingpolycrystalline silicon is then formed on the sidewall of a gateelectrode by a sidewall leaving method and the end of the reading gateis substantially protruded outward by the sidewall.

Other portions are also formed in the same manner as those in the firstembodiment.

With such a structure, similarly, it is possible to prevent a readingvoltage from being raised by trapping a hot electron.

Next, a manufacturing process will be briefly described. FIGS. 5( a) to5(d) show the manufacturing process. The same formation as that in thefirst embodiment is carried out until a patterning step shown in FIG. 5(b), and the gate electrode 18 and the gate oxide films 15, 16 and 17 areformed as shown in FIG. 2( b). At this time, the pattern of the gateelectrode 18 to be actually the reading gate is formed to be smaller byapproximately 0.2 μm than that in the first embodiment.

As shown in FIG. 5( c), then, SiH₄ is thermally decomposed at 600 to650□ by a low pressure CVD method, thereby forming a high concentrationdoped polycrystalline silicon film 4 b having a thickness of 0.4 μm.

By reactive ion etching having conditions set to have anisotropy, then,a sidewall is left to form the side wall 20 comprising polycrystallinesilicon as shown in FIG. 5( d).

Thereafter, a surface is oxidized so that a solid-state image pick-updevice shown in FIG. 4 is formed.

By this method, similarly, it is possible to employ the same structureas that in the case in which a reading gate is substantially protrudedby the presence of the sidewall 20, and a hot electron can be preventedfrom being trapped into silicon nitride. Thus, it is possible to form asolid-state image pick-up device having a high reliability.

Moreover, the solid-state image pick-up device can be formed by only thelow pressure CVD method to be a low temperature step and an anisotropicetching step without increasing a photolithographic step. Consequently,it is possible to provide a solid-state image pick-up device as designedwithout an increase in a diffusion length.

Fourth Embodiment

Next, a fourth embodiment of the invention will be described withreference to FIG. 6. In the embodiment, a region to be the end of areading gate which is formed by the sidewall of polycrystalline siliconin the third embodiment is formed by a shielding electrode 21 comprisinga tungsten film for covering the periphery of an original reading gate.

If the tungsten film is desirably formed through tungsten silicide, anadhesion can also be enhanced.

In a manufacturing process, a photolithographic step for defining theedge of the shielding electrode is required in place of anisotropicetching for forming a sidewall. Other portions are formed in the samemanner as those in the third embodiment.

In the solid-state image pick-up device, it is very effective to carryout the covering with the shielding film except for a light receivingregion in order to prevent a malfunction. With such a structure,effective advantages can be produced in respect of a shielding propertyand the prevention of trapping of a hot electron.

In the embodiment, the reading gate has been described. In the case inwhich the reading gate is provided integrally with a transfer gate, itis preferable that silicon nitride provided on the edge of the gateelectrode thus integrated should be retreated.

Moreover, while only the edge has a single layer structure in theembodiment, it is also possible to employ a structure in which the wholegate oxide film of the gate electrode such as a reading gate which isthe closest to a photoelectric converting section does not contain thesilicon nitride.

Furthermore, a polycrystalline silicon film or a metallic film may beused in the shape of the shielding film. In this case, even if the filmdoes not have the shielding property, the function of the electrode 21can be fulfilled. Moreover, tantalum, titanium, molybdenum and cobalt aswell as tungsten may be used for the metallic film.

As described above, the solid-state image pick-up device according tothe invention has such a single layer structure that the gate oxide filmof the transfer path of a charge coupled device (CCD) which is close toa photoelectric converting section does not contain a silicon nitridefilm on an end at the photoelectric converting section side, and thesilicon nitride film is not present on the end of an electrode on whichan electric field converges most greatly in the application of a readingpulse. Consequently, it is possible to maintain an excellent readingcharacteristic without causing the aging of a voltage to be applied to areading gate.

According to the invention, moreover, it is possible to very easilyprovide a method of manufacturing a solid-state image pick-up devicehaving an excellent reading characteristic which does not cause theaging of a gate voltage.

1. A method of manufacturing a solid-state image pick-up device,comprising the steps of: forming a gate oxide film having a laminationstructure including at least a silicon nitride film and a silicon oxidefilm on an insulating film of a surface of a semiconductor substrate;forming a charge transfer electrode on the gate oxide film; etching thegate oxide film by using the charge transfer electrode as a mask; andcarrying out etching on an etching condition that an etching selectiveratio of the silicon nitride film to the silicon oxide film is high,thereby selectively removing an end of the silicon nitride film.
 2. Themethod of manufacturing a solid-state image pick-up device according toclaim 1, wherein the selective removing step is a chemical dry etching(CDE) step.
 3. The method of manufacturing a solid-state image pick-updevice according to claim 1, wherein the selective removing step is anisotropic etching step using phosphoric acid.
 4. A method ofmanufacturing a solid-state image pick-up device, comprising the stepsof: forming a gate oxide film having a lamination structure including atleast a silicon nitride film and a silicon oxide film on an insulatingfilm of a surface of a semiconductor substrate; forming a chargetransfer electrode having a sidewall on the gate oxide film; etching thegate oxide film to form a gate oxide film sidewall adjacent the chargetransfer electrode sidewall; and etching the gate oxide film sidewall toform an opening in the silicon nitride layer extending beneath thecharge transfer electrode.
 5. The method of claim 4 wherein said step ofetching the gate oxide film sidewall comprises the step of etching thegate oxide film sidewall using a process wherein the selective ratio ofsilicon nitride film to silicon oxide film is high.
 6. The method ofclaim 4 wherein said step of etching the gate oxide film sidewallcomprises the step of selectively etching the silicon nitride portion ofthe gate oxide film sidewall.
 7. The method of claim 4 including theadditional step of filling the opening with silicon oxide.
 8. The methodof claim 4 wherein said step of etching the gate oxide film to form agate oxide film sidewall comprises the step of etching the gate oxidefilm using the charge transfer electrode as a mask.
 9. The method ofclaim 4, wherein said step of etching the gate oxide film sidewallcomprises the step of chemical dry etching (CDE) the gate oxide filmsidewall.
 10. The method of claim 4 wherein said step of etching thegate oxide film sidewall comprises the step of isotropically etching thegate oxide film sidewall using phosphoric acid.